Plant Hormones Flashcards
What are meristems?
Regions of cells capable of mitotic division and growth in plants
What are the two types of growth that can contribute to an increase in the size of a plant?
- Primary growth:
- occurs in apical meristem of buds and root tips
- increases height of plant - Secondary growth:
- increases plant girth and occurs in the lateral meristem in the stem
What are three types of primary meristem?
Procambium - vascular tissues
Protoderm - epidermis
Ground meristem - ground tissue
What do the three transitional meristems give rise to?
The root’s primary tissue systems - epidermis, vascular tissues, ground tissues
Why would plants need hormones?
- controlling growth
- responses to environmental changes
- controlling reproductive cycles
- responses to competition for resources
What are two classes of plant hormones? Give examples.
Growth promoters: Auxins, Gibberellins, Cytokinins
Growth inhibitors: Ethylene gas, Abscisic acid
How can hormones promote plant growth?
Two ways:
1. Stimulating elongation in cells
- Stimulating cell division in meristems to produce new cells
What are the roles of Auxin?
Tropisms
Apical dominance
Growth of adventitious roots
Fruit growth
What is tropism?
Tropisms are the growth of a plant toward or away from a stimulus, including: Phototropism: in response to light Gravitropism: in response to gravity Thigmotropism: in response to touch
What does tropism do?
Tropisms involve cell elongation or suppression of cell elongation on one side of a plant, causing the plant to grow in a particular direction
What is phototropism?
If the tip is exposed to light on one side, the IAA drifts to the darker side. This prompts that side to grow more (the cells elongate), bending the tip towards the light source.
What is gravitropism (geotropism)?
Auxin concentrates in cells on lower side,
causing differential growth
Roots: cells on lower side are inhibited by auxin, root points downwards towards gracity
Shoots: cells on lower side are stimulated by auxin, shoot points upwards
How do cells elongate?
Auxin stimulates proton pumps pump protons (H+) into cell wall expansins modify hydrogen bonds between cellulose molecules molecules slide past one another, allowing for elongation
What happens when plant cells are illuminated by light from one direction?
Transport proteins in plasma membrane on the shaded side of cell are activated and auxin is transport to the shaded side of plant
Why do shoots bend upward?
Auxins are more concentrated on the lower side of stem, causing the cells there to elongate
What causes the roots to bend downward?
Auxin concentration on the lower side of the root suppresses cell elongation. The upper side continue to grow and roots bend downward
How does a root know which way is down?
Plastids in root cap cell tend to settle on the bottom side of cell and stimulates the release of auxins at the bottom
What helps stimulate lateral buds even if the shoot tip is removed?
Cytokinins
Why does a plant grows into a tall slender shape?
When the apical bud grows more quickly than the lateral buds
What happens if the apical bud is pruned?
The lateral buds are no longer inhibited and the plant grows into a low bushy shape.
What are adventitious roots?
Adventitious roots are those growing out of places where roots don’t normally grow. Auxins stimulate root growth on the end of a cutting.
What is the correlation between auxins and fruit growth?
- developing seeds produce auxins that stimulate growth of plant ovary into fruit
- removal of seeds from a strawberry prevents the fruit from growing
What are gibberellins?
Hormones that cause seedlings to grow excessively tall and t=fall over
What are the functions of gibberellins?
Promotes cell elongation in the internodes of
plants.
Stimulates growth of the ovary wall into a
fruit.
Stimulates seed germination and release of
food reserves in seeds.
What are the functions of cytokinins?
Promote growth of lateral buds when auxin
concentrations are low.
Promote cell division in meristems.
Stimulate fruit and seed development.
Delays senescence of plant parts. (senescence: loss of cells power to grow and divide)
What are the functions of ethylene?
Released by fruits and causes the fruits to ripen faster. Causes plant parts to age and die (senescence). Inhibits stem elongation.
What are the functions of abscisic acid?
Controls seed and bud dormancy.
Inhibits gibberellins.
Promotes senescence in plants.
Why do flowers grow?
Changes in gene expression in the shoot apex causes the meristem to produce flowers instead of leaves
What are the roles of auxins and cytokinins in the roots?
Auxins: gravitropism in roots
Cytokinins: Ensure roots and shoots grow at equal rates
How does abscisic affect water loss?
It controls the closing of stomata and hence regulates the water loss
What do abscisic acid and gibberellins do for a seed?
Acid: seed dormancy
G: triggers germination in dormant seeds
What does auxins and gibberellins do at the shoot?
Auxins: promote apical dominance and primary cell growth
G: causes stem elongation by promoting cell elongation and cell division
What is the role of cytokinins, auxins, and ethylene in fruit growth?
Cytokinins: stimulating fruit growth
Auxins: promote fruit growth but inhibit ripening
Ethylene: ripening of certain fruit
What is the role of ethylene and abscisic acid in leaves?
E: loss of leaves and death of flowers
Acid: initiates stress responses in plants
